The present invention relates generally to the field of flue gas cleanup methods and apparatus and, in particular, to a method for removing mercury from the flue gas generated during the combustion of fossil fuels or solid wastes, through the use of a chelating agent.
In recent years, the U.S. Department of Energy (DOE) and the U.S. Environmental Protection Agency (EPA) have supported research to measure and control the emissions of Hazardous Air Pollutants (HAPs) from coal-fired utility boilers. The initial results of several research projects showed that the emissions of heavy metals and volatile organic carbons (VOCs) are very low, except for mercury (Hg). Unlike most of the other metals, most of the mercury remains in the vapor phase and does not condense onto fly ash particles at temperatures typically used in electrostatic precipitators and fabric filters. Therefore, it cannot be collected and disposed of along with fly ash like the other metals. To complicate matters, mercury can exist in its oxidized (Hg.sup.+2) or elemental (Hg.sup.0) form and each is affected differently by subsequent downstream pollution control equipment. In a conventional wet scrubber Hg.sup.+2 is relatively easy to capture while capturing Hg.sup.0 is difficult. The relative amount of each species appears to depend on several factors such as fuel type, boiler combustion efficiency, the type of particulate collector installed, and several other factors. As for the type of particulate collector installed, it has been shown that an electrostatic precipitator (ESP), as is used in the majority of utility applications, affects the process chemistry so that Hg.sup.+2 is converted to Hg.sup.0 within a downstream wet scrubber, also commonly used in utility applications to reduce SO.sub.2 emissions. The Hg.sup.0 is then emitted with the flue gas.
Most of the recent efforts to capture and remove mercury from flue gas have concentrated on gas-phase reactions with introduced reagents such as activated carbon.
The subject of mercury emissions by the utility industry is a new area being investigated by both the DOE and EPA.